Two-stroke, multi-cylinder engines have been plagued with problems of misfiring and generally rough running at idle and light loads. The principle cause of misfires is the extreme dilution of the fresh fuel charge with exhaust products. Because there is so little combustible mixture in the cylinder, the probability of ignition is extremely small. In addition, and particularly in multi-cylinder engines having interconnected exhaust ducts, there are pressure waves which, while highly beneficial at high speeds, interfere with the scavenging process at low speeds.
Various attempts have been made in the past to overcome misfiring and rough running at idle and low speeds. For example, it has been proposed to interrupt the ignition in one or more cylinders by grounding the spark plug associated with the cylinders. In another proposal, one or more cylinders are retarded in throttle advance progression so as to reduce power on that cylinder unitl a given speed is reached. However, none of these proposals have been entirely satisfactory, for they tend to produce an abrupt transition as the firing of cylinders is interrupted or re-established.